If you hang out on the internet long enough, you'll probably see a 1990s Nintendo store poster designed to correct how users use the word "Nintendo" correctly.
While the ad gives the impression that Mario is giving you a grammar lesson, when the image appeared on the internet, it was simply treated as Nintendo's usual pompous publicity. It was seen by netizens as the company's arrogance in telling its fans what they can and can't do.
However, is this really the case? At least it does have a more plausible reason to exist.
Some people may feel that calling all games "Nintendo" is very advantageous for Nintendo, after all, it is free advertising and can stand out from the competition. Although it has been more widely publicized, in fact, in the world of trademark law, this is actually the worst situation for a company.
In general, to register a trademark of a product, service or company, its name must be a highly specific word, or a word used to describe it. For example, you can never trademark the term "video game" because it's not only a very broad term, but also part of a public domain dictionary, a phrase that everyone uses to describe that type of media.
Unfortunately, however, for businesses, languages are constantly evolving and expanding. Even over the past two decades, multiple product names have become part of people's everyday language. Think back to when you smoke, would you use "Lai Gen Huazi" to refer to smoking? When you make a presentation slide, do you say "make a PPT"? When you see a fake image online, do you say "it's PS"? Or when searching for information, you say "go to Baidu"?
Therefore, when a trademark word becomes part of a dictionary of the public domain, it is considered "generalized". When this happens, anyone can legitimately challenge its right to use.
For example, in 2019, Apple lost trademark rights to the terms "App" and "App Store" after being challenged by Amazon in court, and Amazon was able to use those phrases on its tablet series.
And even for big brands like Coca-Cola, the trademark "Coke" is currently in jeopardy, because more and more people will order "Coke" directly in restaurants, even if they want other brands of Coke drinks instead of Coca-Cola. The term has evolved into a noun to describe drinks.
Therefore, because Nintendo was worried that such a trademark might enter the public domain at that time, there would be such an advertisement.